Using boost::spirit, how do I require part of a record to be on its own line?

As the article states, the second parameter is not meaningful unless the expression is part of a rule, so lets start with the third. A placeholder for the second parameter is still needed though and for this use boost::fusion::unused_type. All this example does is switch the match to a non-match, which is reflected in the parser output.

According to hkaiser, in boost 1.44 and up setting the match flag to false will cause the match to fail in the normal way. If alternatives are defined, the parser will backtrack and attempt to match them as one would expect. However, in boost To see this, add phoenix include boost/spirit/include/phoenix.

The 6 is the first digit of the second int in the input which indicates the alternative is taken using the skipper and without backtracking. Notice also that the match is considered succesful, based on the alternative. Once boost 1.44 is out, the match flag will be useful for applying match criteria that might be otherwise difficult to express in a parser sequence.

Note that the match flag can be manipulated in phoenix expressions using the _pass placeholder. The more interesting parameter is the second one, which contains the qi-phoenix interface, or in qi parlance, the context of the semantic action. The context parameter embodies the Attribute, Arg1, ... ArgN, and qi::locals template paramters, wrapped in a boost::spirit::context template type.

This attribute differs from the function parameter: the function parameter attribute is the parsed value, while this attribute is the value of the rule itself. A semantic action must map the former to the latter.